(1) Field of the Invention
The present invention relates to a hydraulic brake device which functions to ensure an appropriate control of brake fluid pressure in a hydraulic circuit connected to wheels of an automotive vehicle.
(2) Description of the Related Art
As disclosed in Japanese Laid-Open Patent Application No. 4-221258, there is known a hydraulic brake device that electrically controls a brake fluid pressure supplied into a wheel cylinder. Such a hydraulic brake device is provided with a master cylinder which generates a brake fluid pressure (hereinafter called the master cylinder pressure P.sub.M/C), and a high pressure source which generates a boosted brake fluid pressure (hereinafter called the accumulator pressure) which is higher than the master cylinder pressure P.sub.M/C.
The master cylinder generates the master cylinder pressure P.sub.M/C in response to a brake pedal pressure exerted on a brake pedal by a vehicle operator. Hence, a braking force that is proportional to the brake pedal pressure is exerted on a wheel by the wheel cylinder when the master cylinder pressure P.sub.M/C is supplied into the wheel cylinder. The high pressure source generates the accumulator pressure which is higher than the master cylinder pressure P.sub.M/C. Hence, an increased braking force that is higher than the brake pedal pressure is exerted on the wheel by the wheel cylinder when the accumulator pressure is supplied into the wheel cylinder. In the latter case, the operator can perform the braking operation more easily than in the former case to effect the braking force on the vehicle.
In the conventional device of the above-mentioned publication, when the hydraulic brake device is operating normally, an electrical brake control is performed so that the master cylinder is separated or cut off from the wheel cylinder and the high pressure source is brought in line with the wheel cylinder. The accumulator pressure from the high pressure source is supplied into the wheel cylinder. In such a configuration, it is possible for the wheel cylinder to exert a large braking force on the vehicle wheel in comparison with the brake pedal pressure. According to the electrical brake control, the wheel cylinder pressure of the wheel cylinder is set at the accumulator pressure.
When a malfunction in the conventional device of the above publication is detected, the electrical brake control is switched to a manual operation mode so that the high pressure source is separated or cut off from the wheel cylinder and the master cylinder is brought in line with the wheel cylinder. The master cylinder pressure P.sub.M/C from the master cylinder is supplied into the wheel cylinder. In such a configuration, when the operator manually exerts the brake pedal pressure on the brake pedal, it is possible for the wheel cylinder to exert a braking force which is approximately equal to the master cylinder pressure P.sub.M/C. Hence, although the malfunction has occurred, the wheel cylinder pressure of the wheel cylinder is set to be almost equal to the master cylinder pressure P.sub.M/C by the manual operation of the operator.
However, in the conventional device of the above publication, when the high pressure source is operating normally but a malfunction in a sensor of an electrical system is detected, the electrical brake control is stopped and the switching to the manual operation mode is performed so that the high pressure source is cut off from the wheel cylinder, and the master cylinder is connected with the wheel cylinder. Only the master cylinder pressure P.sub.M/C from the master cylinder is supplied into the wheel cylinder. In such a configuration, the accumulator pressure from the high pressure source cannot be supplied into the wheel cylinder even if the high pressure source is operating normally. The level of the braking force exerted on the wheel in such a case is proportional to the brake pedal pressure on the brake pedal by the operator. In order to exert a large braking force on the wheel, the vehicle operator must depress the brake pedal very heavily.
Practically, there exist many cases in which the accumulator pressure from the high pressure source can be utilized to exert an increased braking force on the vehicle wheel even when a malfunction in an electrical system is detected. However, the conventional device of the above publication does not supply the accumulator pressure from the high pressure source into the wheel cylinder when a malfunction in the electrical system is detected. It is difficult for the conventional device of the above publication to ensure an appropriate control of the brake fluid pressure when such a malfunction is detected.